The present invention is concerned with a method for preparing trichloroethylene, starting with 1,2-dichloroethane, wherein the recirculation of incompletely reacted starting materials is considerably reduced.
The state of the art of trichloroethylene production may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", Second Edition, Vol. 5 (1964), pp. 183-195, under the section TRICHLOROETHYLENE, particularly page 190 where the use of 1,2-dichloroethane as a starting material is mentioned. According to Kirk-Othmer, British Patent Nos. 904,405 and 913,040 disclose that starting with 1,2-dichloroethane, the chlorination may be effected, at 380.degree.-510.degree.C, by a mixture of air and chlorine in the presence of a copper chloride or copper-potassium chloride catalyst. U.S. Pat. No. 3,029,299 of Thermet et al which issued Feb. 3, 1958, discloses that the 1,2-dichloroethane can be converted to trichloroethylene by chlorination in a fluidized bed, at a high flow rate, and at 350.degree.-450.degree.C. According to British Pat. No. 904,084 various C.sub.1 to C.sub.4 chlorinated and unchlorinated aliphatic-hydrocarbon mixtures are chlorinated to trichloroethylene by chlorine or a hydrogen chloride and oxygen mixture in a fluidized bed, at 290 -500.degree.C, using copper chloride or copper-zinc chloride catalyst.
U.S. Pat. 3,631,207 of Kircher, Jr. et al which issued December 28, 1971, discloses the dehydrochlorination of 1,1,2,2,-tetrachloroethane, 1,1,2,2,-tetrachloroethane, pentachloroethane or mixtures thereof at an elevated temperature of about 145.degree.C to below 300.degree.C in a liquid state while under positive pressure of about 35 p.s.i.a. to 300 p.s.i.a. in the presence of activated carbon to form trichloroethylene, tetrachloroethane or mixtures thereof.
Trichloroethylene is a significant aliphatic chlorinated hydrocarbon. Because of its excellent solubility, it is a preferred means for metal degreasing and is used in chemical cleaning.
It is known to prepare trichloroethylene from acetylene by a variety of processes. However, in view of the high cost of high-energy acetylene, processes have been recently developed, which start from ethane, ethylene or their chlorinated derivatives. One may distinguish between two groups in these recent processes, namely those which lead directly to trichloroethylene at high temperatures and in the gaseous state in purely thermal or also catalytic manner, and those which first chlorinate ethylene in the liquid phase to tetrachloroethane and subsequently convert the latter by means of cracking in the gaseous or liquid phase to trichloroethylene (the 2-stage process). The 2-stage process is generally advantageous with respect to the gaseous process in that there is higher selectivity and simpler removal of the heat of chlorination by partial evaporation of the reagent mixture.
It is known that the substituent chlorination of saturated, organic compounds is much accelerated by the simultaneous presence of chlorine-adding olefins as disclosed in U.S. Pat. No. 1,991,600 of Deanesly, which issued Feb. 19, 1935. Recently this effect has also been applied in various processes for preparing trichloroethylene. For instance, 1,2-dichloroethane is chlorinated in the presence of ethylene to a mixture of chlorinated hydrocarbons containing predominantly tetrachloroethanes, the latter being separated and lending themselves to conversion into trichloroethylene by means of dehydrochlorination.
U.S. Pat. No. 3,631,207 discloses, among other things, a process for preparing trichloroethylene in some cases together with tetrachloroethylene, wherein chlorine and ethylene are reacted in a liquid medium at 0.degree.-250.degree.C, the medium consisting of chloroethanes with at least 2.5 atoms of chlorine per molecule. Subsequently at least part of the chloroethanes are removed, and the product removed is separated into a fraction of higher boiling point containing predominantly chloroethanes with 4 or more chlorine atoms per molecule, and into a lower boiling point fraction of average chlorine content less than that of the first fraction. At least part of the lower boiling-point fraction then is used as the liquid medium for the above-described chlorination reaction, while the higher boiling-point fraction containing predominantly tetra- and pentachloroethanes, is dehydrochlorinated under pressure and in the presence of activated carbon, being in the liquid state and subjected to an elevated temperature of 190.degree.-250.degree.C. Another process (INDUSTRIAL & ENGINEERING CHEMISTRY, Vol. 62, (1970), No. 5, pp. 36-41) in a similar manner chlorinates ethylene in the liquid phase at temperatures from 100.degree. to 130.degree.C and 10 atm. pressure. Subsequently the reaction product is separated into a low boiling-point fraction of dichloroethane and 1,1,2-trichloroethane and into a high boiling-point fraction of tetrachloroethane and pentachloroethane. Again the low boiling-point products are used as the reaction medium for chlorinating ethylene. The tetrachloroethanes and pentachloroethanes are thermally cracked at 450.degree.-500.degree.C under pressure into trichloroethylene and perchloroethylene, with ferric chloride being used in catalytic amounts.
French Pat. No. 1,587,362 discloses how to chlorinate 1,2-dichloroethane in the presence of chlorine derivatives of ethylene, i.e. vinyl chloride, cis-trans-1,2-dichloroethylene and vinylidene chloride (1,1-dichloroethylene), possibly with mixtures therefrom. The catalytic effect of the chlorine derivatives on the substituent chlorination of 1,2-dichloroethylene however is less pronounced than that of ethylene, so that large quantities of 1,2-dichloroethylene must be circulated or cycled, and the space-time yields are appreciably less than when using C.sub.2 H.sub.4 as a catayst. Following the separation of the 1,2-dichloroethylene from the reaction product, one obtains a mixture of 1,1,2-trichloroethane, tetrachloroethanes and pentachloroethanes, which mixture is dissociated pyrolytically at 450.degree.C, forming vinylidene chloride, cis-trans-1,2-dichloroethylene, trichloroethylene and perchloroethylene. The products are condensed from the hydrogen chloride that was produced simultaneously and the mixture thus obtained is subjected to distillation-separation into a low boiling-point fraction consisting of vinylidene chloride and cis-trans-1,2-dichloroethylene and into a high boiling-point fraction consisting of trichloroethylene and perchloroethylene. The low boiling-point compounds of vinylidene chloride and cis-trans-1,2-dichloroethylene are fed back of chlorinating 1,2-dichloroethane.
However, the above-mentioned and commerically disclosed 2-stage process suffers from the drawback of a relatively high perchloroethylene content in the product and of high expenditures of economical and commercial nature because of the recirculation required for the incompletely reacted feed stock materials: in increasing degree, perchloroethylene is manufactured by means of chlorinating pyrolysis, that is, chlorinating at 600.degree.C with thermal fission or dissociation of the chemical compound, from wastes of chlorinated hydrocarbons. Such a process is becoming increasingly significant in highly industrialized countries because of ecological considerations. Therefore, additional accumulation or output of perchloroethylene is increasingly undesirable and unprofitable.
Now, besides being generated by chlorinating C.sub.2 compounds with 4 or fewer chlorine atoms in the molecule in the gaseous phase, perchloroethylene may also be quickly and quantitatively obtained by dehydrochlorinating the pentachloroethane obtained from liquid-phase chlorination of C.sub.2 H.sub.4 and/or 1,2-dichloroethane. In order to keep the pentachloroethane content low in the liquid-phase chlorination, the latter must proceed with low conversion rates of the reagents, so the recirculation will be high and hence the costs are considerable.
High recirculation rates also occur when dehydrochlorinating pure tetrachloroethanes, because full conversions may not be undertaken if high yields of trichloroethylene and/or avoidance of soot precipitation from the hot walls (an endothermic reaction) are desired. The cycling operation of unreacted feed stock materials cannot take place unless substantial expenditures are met, in the case of preparing trichloroethylene and perchloroethylene, because the boiling points of perchloroethylene and of 1,1,1,2-tetrachloroethylene are close to one another, 121.degree.C and 120.degree.-130.degree.C, respectively. Furthermore, the recirculated product must be freed from minute amounts of materials with higher boiling points, predominantly C.sub.4 chlorine derivatives, because the latter will enrich the feed and cause interferences. Such separation, too, is expensive, because the boiling points of the recovered initial materials are very high for some components of the mixture (159.degree.C for pentachloroethane).
Essentially the drawbacks discussed above are caused by the attempts in the classical manner, similar to the acetylene process, to obtain symmetrial and/or asymmetrical tetrachloroethane by starting from C.sub.2 H.sub.4 and/or 1,2-dichloroethane, the tetrachloroethane being cracked in a known pyrolytic manner.